Data obtained with cultured eukaryotic cells and in vivo tissues implicate the cyclic nucleotides, cyclic AMP and cyclic GMP, in the regulation of the cell cycle. However, the results and general conclusions are controversial and ambiguous. A major problem is the lack of natural synchrony and the possible perturbations of the cell cycle by techniques used to experimentally derive synchrony. The objective of the proposed research is to determine if cyclic AMP and cyclic GMP modulate the traverse of the cell cycle in Physarum polycephalum. Physarum polycephalum is an acellular slime mold which can grow as a macroplasmodium possessing a naturally synchronous cell cycle. The specific cell cycle events which will be studied are the synthesis of DNA and mitosis. The cellular levels of cyclic AMP and cyclic GMP during the cell cycle will be measured by radioimmunoassay. The cellular levels of cyclic nucleotides will be changed to determine if the traverse of the cell cycle is altered. Substances used to change the cyclic nucleotide levels will be added to the growth media or microinjected into macroplasmodia at different times during the cell cycle. The timing and duration of each stage of the cell cycle and the levels of the cyclic nucleotides will be measured after these treatments and compared to control macroplasmodia. The enzymatic activities of adenylate and guanylate cyclases, the phosphodiesterases, and the cyclic nucleotide-dependent protein kinases will be measured during the cell cycle. The subcellular localization of the cyclic nucleotides in the macroplasmodium during the cell cycle will be determined by immunofluorescent microscopy. The proposed experimentation will determine if the cycle nucleotides modulate the cell cycle of Physarum polycephalum; at what phase(s) of the cycle such modulation occurs; and if the modulation is by inhibition, stimulation, or if both types of regulation are involved depending on the phase of the cell cycle.